the september before your child's 5th birthday. (or the september after your child's 4th birthday). if they turn 4 from sep 08 to aug 09 they will start in sep 09. applications now for that school year. hope that helps.
Legally a child has to be in education (school or otherwise) by the term after they turn 5y.
In most state schools in England, children start school in teh year they turn 5. Many schools have one intake - so all children start in the September before they are 5. Some have a two intake system, so children born after Mar 31 and before Sep 1 start nt he January before they turn 5.